You cannot subtract cloud layers from each other, they do not produce color output, they are for volumetric shading only. You probably need to get a better understanding of what the different colored nodes are. Red are your Shader nodes, they generally output Color and/or Displacement, and can be combined, subtracted, etc. Blue are Function Nodes, they output data specific to their name/category, e.g. "Constant Scalar" outputs scalar data. Blue nodes are atmosphere nodes, they do not output the same kinds of data as other nodes and they only work when plugged-in to the atmosphere input of the planet. Green nodes are Heightfield nodes, they only work when fed into a Heightfield node.
So you need to work on the noise functions (shader and/or function nodes) themselves first, add and subtract, and otherwise manipulate those, then feed them into your cloud layer(s) to volumetrically shade the shapes you have created.
Lots of cloud types are possible. Some examples are provided in the Terragen Presets Pack 1 (in Free Downloads on our website), as well as by NWDA, Luc Bianco, and others on their respective sites.
Easy Clouds are intended to be just that: easy ways to make the exact cloud type specified. They are not intended to be as versatile, powerful, or flexible as regular cloud nodes. If you want a more powerful cloud node that still uses the new v3 shading model, create a Generic cloud layer.
- Oshyan